A major purpose of conventional patterning techniques of nanowires is to position the nanowires at desired locations. However, conventional patterning techniques use indefinite nanowire networks, thus causing damage to the stability of nanowire devices fabricated using the nanowires. Under these circumstances, recent studies on the patterning of nanowires have focus on the alignment of the nanowires as well as the control over the location of the nanowires. Various processes have been developed to date for patterning nanowires. Of these, a great deal of research has been conducted on printing processes by which nanowires can be patterned over a large area in an economically feasible and simple manner. Studies on the patterning of nanostructures using printing-transfer processes developed hitherto are broadly classified into two types.
The first type of studies have focused on the patterning of nanowires in order to induce selective adsorption of the nanostructures on a surface having not only an affinity but also a repulsive interaction with the nanowires using a printing process. To this end, a substrate is patterned with different functional groups, and then the patterned substrate is exposed to nanowires so as to allow the nanowires to be selectively adsorbed to the intended locations, which are pre-designed for the attachment of the nanowires, of the substrate (Korean Patent Publication No. 10-2007-0112733). This process employs a self-assembly method, but has many difficulties in processing a multilayer of nanowires rather than patterning a monolayer of nanowires and the procedure is also complicated.
The second type of studies relate to the patterning of nanowires by transferring nanowires to a stamp and printing a pattern of the nanowires on a substrate. These second type of studies can be achieved by the following techniques. The first technique is to transfer nanostructures produced on a substrate to a stamp using a known semiconductor manufacturing process and to transfer the nanostructures on the stamp to another substrate where a nanowire device is to be fabricated by printing (Science 314, 1754 (2006), Korean Patent Publication No. 10-2007-0037484). The second technique is to align and grow nanowires, transfer the grown nanowires to a stamp, and print the pattern of the nanowires on a substrate (Nature Nanotech. 2, 230 (2007)). The third technique is to align nanowires in a solution state using a flow of fluid or tension and to transfer the aligned nanowires to a substrate by direct transfer or printing.
Most of the techniques for nanowire synthesis developed hitherto can be largely divided into two types of techniques: i.e. techniques using sol-gel synthesis in a solution state and chemical vapor deposition. The sol-gel synthesis is superior to chemical vapor deposition from the viewpoint of mass production and economic efficiency. The aforementioned first and second techniques associated with direct transfer processes cannot use nanowires produced by sol-gel synthesis. Further, these techniques are dependent on known semiconductor manufacturing processes, incurring considerable production costs. The third technique is effective in aligning various kinds of nanowires over a large area, but is not suitable for controlling the location and length of nanowires.
Meanwhile, vanadium pentoxide (V2O5) particles can be used as active materials for secondary batteries due to their high electrochemical activity. Nanometer-sized vanadium pentoxide can find application in various fields, including catalysts, electrochromic devices, sensors, antistatic coatings, and nanowires of semiconductor circuits. Many methods have been employed to prepare vanadium pentoxide particles. For example, vanadium pentoxide particles are prepared via thermal decomposition of ammonium vanadate (‘a solid-phase method’). According to the solid-phase method, vanadium pentoxide particles are easy to prepare, but they disadvantageously are irregular in shape and are as very large as several micrometers in size. Vapor-phase synthesis is a method in which a heat source such as laser or plasma is used to prepare vanadium pentoxide particles. The vapor-phase method has a difficulty in controlling the processing steps and is economically disadvantageous. In contrast, sol-gel synthesis is a method in which nanometer-sized plate- or ribbon-like vanadium pentoxide particles are prepared using a solution (sol) of a vanadium pentoxide powder in an appropriate solvent. According to the sol-gel method, vanadium pentoxide particles are prepared in a simple and economically feasible way, and fine nanometer-sized vanadium pentoxide nanowires can be prepared. Another sol-gel method is known in which ammonium metavanadate, a specified amount of an acidic ion exchange resin and distilled water are mixed together to prepare fine nanometer-sized vanadium pentoxide nanowires.
The present inventors have succeeded in developing a method for producing a vanadium pentoxide nanowire film using a Langmuir-Blodgett trough (LB trough) (see Korean Patent Application No. 2006-130474). According to this method, a solution of vanadium pentoxide nanowires and a solution of a dioctadecyldimethylammonium halide are used to produce a uniform, high-density film of the vanadium pentoxide nanowires. However, the fabrication of a high-precision device using a highly uniform and dense film of vanadium pentoxide nanowires produced by the method requires further improvements in the characteristics and reproducibility of the device.
Thermal chemical vapor deposition (thermal CVD), RF plasma CVD, microwave plasma CVD, DC plasma CVD, modified catalyst CVD, arc discharge, laser deposition, pyrolysis and vapor-phase growth methods have been used for the preparation of nanowires. According to these methods, nanowires are grown and simultaneously aligned on substrates, or nanowires are capped and forced to be aligned by chemical adsorption using barriers of Langmuir-Blodgett systems.
However, the conventional methods have problems in that it is not easy to align nanowires, to transfer once aligned nanowires to another substrate, and to cut nanowires to desired lengths. In addition, the conventional methods are not applicable to the alignment of nanowires prepared on a large scale by sol-gel synthesis. Particularly, since chemical adsorption techniques using capping materials may greatly affect the electrical properties of nanowires and depend on physical forces of barriers, they have limitations in that nanowires must have a large diameter, a specific rigidity, high aspect ratio, etc.